July 21, 2011

We still have a lot of ground to cover on pointers, but before we do, we need to address certain conceptual frameworks missing from C# that one must be intimately familiar with when moving to C++.

Specifically, in C# you mostly work with the Heap. The heap is not difficult to understand - its a giant lump of memory that you take chunks out of to allocate space for your classes. Anything using the new keyword is allocated on the heap, which ends up being almost everything in a C# program. However, the heap isn't the only source of memory - there is also the Stack. The Stack is best described as what your program lives inside of. I've said before that everything takes up memory, and yes, that includes your program. The thing is that the Heap is inherently dynamic, while the Stack is inherently fixed. Both can be re-purposed to do the opposite, but trying to get the Stack to do dynamic allocation is extremely dangerous and is almost guaranteed to open up a mile-wide security hole.

I'm going to assume that a C# programmer knows what a stack is. All you need to understand is that absolutely every single piece of data that isn't allocated on the heap is pushed or popped off your program's stack. That's why most debuggers have a "stack" of functions that you can go up and down. Understanding the stack in terms of how many functions you're inside of is ok, but in reality, there are also variables declared on the stack, including every single parameter passed to a function. It is important that you understand how variable scope works so you can take advantage of declaring things on the stack, and know when your stack variables will simply vanish into nothingness. This is where { and } come in.

{ and } define scope. Anything declared inside of them ceases to exist outside, but is still accessible to any additional layers of scope declared inside of them. This is a way to see your program's stack in action. When bunny is declared, its pushed on to the stack. Then we enter our first scope area, where we push carrot and lettuce on to the stack and set bunny to 2, which is legal because bunny is still on the stack. When the scope is then closed, however, anything declared inside the scope is popped from the stack in the exact opposite order it was pushed on. Unfortunately, compiler optimization might change that order behind the scenes, so don't rely on it, but it should be fairly consistent in debug builds. First lettuce is de-allocated (and its destructor called, if it has one), then carrot is de-allocated. Consequently, trying to set carrot to 2 outside of the scope will result in a compiler error, because it doesn't exist anymore. This means we can now declare an entirely new integer variable that is also called carrot, without causing an error. If we visualize this as a stack, that means carrot is now directly above bunny. As we enter a new scope area, lettuce is then put on top of carrot, and then grass is put on top of lettuce. We can still assign either lettuce or carrot to bunny, since they are all on the stack, but once we leave this inner scope, grass is popped off the stack and no longer exists, so any attempt to use it causes an error. lettuce, however, is still there, so we can assign lettuce to bunny before the scope closes, which pops lettuce off the stack.

Now the only things on the stack are bunny and carrot, in that order (if the compiler hasn't moved things around). We are about to leave the function, and the function is also surrounded by { and }. This is because a function is, itself, a scope, so that means all variables declared inside of that scope are also destroyed in the order they were declared in. First carrot is destroyed, then bunny is destroyed, and then the function's parameters argc and argv are destroyed (however the compiler can push those on to the stack in whatever order it wants, so we don't know the order they get popped off), until finally the function itself is popped off the stack, which returns program flow to whatever called it. In this case, the function was main, so program flow is returned to the parent operating system, which does cleanup and terminates the process.

You can declare anything that has a size determined at compile time on the stack. This means if you have an array that has a constant size, you can declare it on the stack:

int array[] = {1,2,3,4}; //Declares an array of 4 ints on the stack initialized to 1,2,3,4

Not only that, but you can declare class instances and other objects on the stack.

Class instance(arg1, arg2); //Calls a constructor with 2 arguments
Class instance; //Used if there are no arguments for the constructor
//Class instance(); //Causes a compiler error! The compiler will think its a function.

In fact, if you have a very simple data structure that uses only default constructors, you can use a shortcut for initializing its members. I haven't gone over classes and structs in C++ yet (See Part 3), but here is the syntax anyway:

All of these declare variables on the stack. C# actually does this with trivial datatypes like int and double that don't require a new statement to allocate, but otherwise forces you to use the Heap so its garbage collector can do the work.

Wait a minute, stack variables automatically destroy themselves when they go out-of-scope, but how do you delete variables allocated from the Heap? In C#, you didn't need to worry about this because of Garbage Collection, which everyone likes because it reduces memory leaks (but even I have still managed to cause a memory leak in C#). In C++, you must explicitly delete all your variables declared with the new keyword, and you must keep in mind which variables were declared as arrays and which ones weren't. In both C# and C++, there are two uses of the new keyword - instantiating a single object, and instantiating an array. In C++, there are also two uses of the delete keyword - deleting a single object and deleting an array. You cannot mix up delete statements!

int* Fluffershy = new int();
int* ponies = new int[10];
delete Fluffershy; // Correct
//delete ponies; // WRONG, we should be using delete [] for ponies
delete [] ponies; // Just like this
//delete [] Fluffershy; // WRONG, we can't use delete [] on Fluffershy because we didn't
// allocate it as an array.
int* one = new int[1];
//delete one; // WRONG, just because an array only has one element doesn't mean you can
// use the normal delete!
delete [] one; // You still must use delete [] because you used new [] to allocate it.

As you can see, it is much easier to deal with stack allocations, because they are automatically deallocated, even when the function terminates unexpectedly. std::auto_ptr takes advantage of this by taking ownership of a pointer and automatically deleting it when it is destroyed, so you can allocate the auto_ptr on the stack and benefit from the automatic destruction. However, in C++0x, this has been superseded by std::unique_ptr, which operates in a similar manner but uses some complex move semantics introduced in the new standard. I won't go into detail about how to use these here as its out of the scope of this tutorial. Har har har.

For those of you who like throwing exceptions, I should point out common causes of memory leaks. The most common is obviously just flat out forgetting to delete something, which is usually easily fixed. However, consider the following scenario:

These situations seem obvious, but they will happen to you once the code becomes enormous. This is one reason you have to be careful when inside functions that are very large, because losing track of if statements may result in you forgetting what to delete. A good rule of thumb is to make sure you delete everything whenever you have a return statement. However, the opposite can also happen. If you are too vigilant about deleting everything, you might delete something you never allocated, which is just as bad:

Clearly, one must be careful when dealing with allocating and destroying memory in C++. Its usually best to encapsulate as much as possible in classes that automate such things. But wait, what about that NULL pointer up there? Now that we're familiar with memory management, we're going to dig into pointers again, starting with the NULL pointer.

Since a pointer points to a piece of memory that's somewhere between 0 and 4294967295, what happens if its pointing at 0? Any pointer to memory location 0 is always invalid. All you need to know is that the operating system does some magic voodoo to ensure that any attempted access of memory location 0 will always throw an error, no matter what. 1, 2, 3, and any other double or single digit low numbers are also always invalid. 0xfdfdfdfd is what the VC++ debugger sets uninitialized memory to, so that pointer location is also always invalid. A pointer set to 0 is called a Null Pointer, and is usually used to signify that a pointer is empty. Consequently if an allocation function fails, it tends to return a null pointer. Null pointers are returned when the operation failed and a valid pointer cannot be returned. Consequently, you may see this:

This is known as a safe deletion. It ensures that you only delete a pointer if it is valid, and once you delete the pointer you set the pointer to 0 to signify that it is invalid. Note that NULL is defined as 0 in the standard library, so you could also say blink = NULL.

Since pointers are just integers, we can do pointer arithmetic. What happens if you add 1 to a pointer? If you think of pointers as just integers, one would assume it would simply move the pointer forward a single byte.

This isn't what happens. Adding 1 to a pointer of type integer results in the pointer moving forward 4 bytes.

Adding or subtracting an integer $i$ from a pointer moves that pointer $i\cdot n$ bytes, where $n$ is the size, in bytes, of the pointer's type. This results in an interesting parallel - adding or subtracting from a pointer is the same as treating the pointer as an array and accessing it via an index.

int main(int argc, char* argv[])
{
int* kitties = new int[14];
int* a = &kitties[7];
int* b = kitties+7; //b is now the same as a
int* c = &a[4];
int* d = b+4; //d is now the same as c
int* e = &kitties[11];
int* f = kitties+11;
//c,d,e, and f now all point to the same location
}

So pointer arithmetic is identical to accessing a given index and taking the address. But what happens when you try to add two pointers together? Adding two pointers together is undefined because it tends to produce total nonsense. Subtracting two pointers, however, is defined, provided you subtract a smaller pointer from a larger one. The reason this is allowed is so you can do this:

int main(int argc, char* argv[])
{
int* eggplants = new int[14];
int* a = &eggplants[7];
int* b = eggplants+10;
int diff = b-a; // Diff is now equal to 3
a += (diff*2); // adds 6 to a, making it point to eggplants[13]
diff = a-b; // diff is again equal to 3
diff = a-eggplants; //diff is now 13
++a; //The increment operator is valid on pointers, and operates the same way a += 1 would
// So now a points to eggplants[14], which is not a valid location, but this is still
// where the "end" of the array technically is.
diff = a-eggplants; // Diff now equals 14, the size of the array
--b; // Decrement works too
diff = a-b; // a is pointing to index 14, b is pointing to 9, so 14-9 = 5. Diff is now 5.
return 0;
}

There is a mistake in the code above, can you spot it? I used a signedinteger to store the difference between the two pointers. What if one pointer was above 2147483647 and the other was at 0? The difference would overflow! Had I used an unsigned integer to store the difference, I'd have to be really damn sure that the left pointer was larger than the right pointer, or the negative value would also overflow. This complexity is why you have to goad windows into letting your program deal with pointer sizes over 2147483647.

In addition to arithmetic, one can compare two pointers. We already know we can use == and !=, but we can also use < > <= and >=. While you can get away with comparing two completely unrelated pointers, these comparison operators are usually used in a context like the following:

Here the for loop increments the pointer itself rather than an index, until the pointer reaches the end, at which point it terminates. But, what if you had a pointer that didn't have any type at all? void* is a legal pointer type, that any pointer type can be implicitly converted to. You can also explicitly cast void* to any pointer type you want, which is why you are allowed to explicitly cast any pointer type to another pointer type (int* p; short* q = (short*)p; is entirely legal). Doing so, however, is obviously dangerous. void* has its own problems, namely, how big is it? The answer is, you don't know. Any attempt to use any kind of pointer arithmetic with a void* pointer will cause a compiler error. It is most often used when copying generic chunks of memory that only care about size in bytes, and not what is actually contained in the memory, like memcpy().

Now that we know all about pointer manipulation, we need to look at pointers to pointers, and to anchor this in a context that actually makes sense, we need to look at how C++ does multidimensional arrays. In C#, multidimensional arrays look like this:

int[,] table = new int[4,5];

C++ has a different, but fairly reasonable stack-based syntax. When you want to declare a multidimensional array on the heap, however, things start getting weird:

int unicorns[5][3]; // Well this seems perfectly reasonable, I wonder what-
int (*cthulu)[50] = new int[10][50]; // OH GOD GET IT AWAY GET IT AWAAAAAY...!
int c=5;
int (*cthulu)[50] = new int[c][50]; // legal
//int (*cthulu)[] = new int[10][c]; // Not legal. Only the leftmost parameter
// can be variable
//int (*cthulu)[] = new int[10][50]; // This is also illegal, the compiler is not allowed
// to infer the constant length of the array.

Why isn't the multidimensional array here just an int**? Clearly if int* x is equivalent to int x[], shouldn't int** x be equivalent to int x[][]? Well, it is - just look at the main() function, its got a multidimensional array in there that can be declared as just char** argv. The problem is that there are two kinds of multidimensional arrays - square and jagged. While both are accessed in identical ways, how they work is fundamentally different.

Let's look at how one would go about allocating a 3x5 square array. We can't allocate a 3x5 chunk out of our computer's memory, because memory isn't 2-dimensional, its 1-dimensional. Its just freaking huge line of bytes. Here is how you squeeze a 2-dimensional array into a 1-dimensional line:

As you can see, we just allocate each row right after the other to create a 15-element array ($5\cdot 3 = 15$). But then, how do we access it? Well, if it has a width of 5, to access another "row" we'd just skip forward by 5. In general, if we have an $n$ by $m$ multidimensional array being represented as a one-dimensional array, the proper index for a coordinate $(x,y)$ is given by: array[x + (y*n)]. This can be extended to 3D and beyond but it gets a little messy. This is all the compiler is really doing with multidimensional array syntax - just automating this for you.

Now, if this is a square array (as evidenced by it being a square in 2D or a cube in 3D), a jagged array is one where each array is a different size, resulting in a "jagged" appearance:

We can't possibly allocate this in a single block of memory unless we did a lot of crazy ridiculous stuff that is totally unnecessary. However, given that arrays in C++ are just pointers to a block of memory, what if you had a pointer to a block of memory that was an array of pointers to more blocks of memory?

Suddenly we have our jagged array that can be accessed just like our previous arrays. It should be pointed out that with this format, each inner-array can be in a totally random chunk of memory, so the last element could be at position 200 and the first at position 5 billion. Consequently, pointer arithmetic only makes sense within each column. Because this is an array of arrays, we declare it by creating an array of pointers. This, however, does not initialize the entire array; all we have now is an array of illegal pointers. Since each array could be a different size than the other arrays (this being the entire point of having a jagged array in the first place), the only possible way of initializing these arrays is individually, often by using a for loop. Luckily, the syntax for accessing jagged arrays is the exact same as with square arrays.

int main(int argc, char* argv[])
{
int** jagged = new int*[5]; //Creates an array of 5 pointers to integers.
for(int i = 0; i < 5; ++i)
{
jagged[i] = new int[3+i]; //Assigns each pointer to a new array of a unique size
}
jagged[4][1]=0; //Now we can assign values directly, or...
int* second = jagged[2]; //Pull out one column, and
second[0]=0; //manipulate it as a single array
// The double-access works because of the order of operations. Since [] is just an
// operator, it is evaluated from left to right, like any other operator. Here it is
// again, but with the respective types that each operator resolves to in parenthesis.
( (int&) ( (int*&) jagged[4] ) [1] ) = 0;
}

As you can see above, just like we can have pointers to pointers, we can also have references to pointers, since pointers are just another data type. This allows you to re-assign pointer values inside jagged arrays, like so: jagged[2] = (int*)kitty. However, until C++0x, those references didn't have any meaningful data type, so even though the compiler was using int*&, using that in your code will throw a compiler error in older compilers. If you need to make your code work in non-C++0x compilers, you can simply avoid using references to pointers and instead use a pointer to a pointer.

This also demonstrates the other use of a pointer-to-pointer data type, allowing you to remotely manipulate a pointer just like a pointer allows you to remotely manipulate an integer or other value type. So obviously you can do pointers to pointers to pointers to pointers to an absurd degree of lunacy, but this is exceedingly rare so you shouldn't need to worry about it.
Now you should be strong in the art of pointer-fu, so our next tutorial will finally get into object-oriented techniques in C++ in comparison to C#.
Part 3: Classes and Structs and Inheritance OH MY!

July 16, 2011

But hey we're all alive and everything eventually worked out. I got on the train and was 7 minutes late but they were like 7 minutes late picking me up after having barely found the train station on time after first having gone to the airport because ?????? So then they got lost trying to get back to the place they had already been to (the airport) because THAT FUCKING GUY WITH THE PINK HAIR (chris) wanted to use his STUPID PHONE instead of USING A GODDAMN MAP LIKE YOU SHOULD. We finally got to the airport and then Sam missed the turn so we almost LEFT the airport until I was like OK JUST FUCKING TURN RIGHT HERE OK? So then we turned around, managed to get into the airport parking, took a tram to the terminal, walked into the nearest door for baggage claim and exactly 3 seconds after walking inside there was a big fat announcement: "WILL SAMUEL BOYD, CHRIS icantremember, AND ERIK MCCLURE PLEASE PICK UP THEIR PASSENGER AT BAGGAGE CLAIM 8?" and we all RAN DOWN TO BAGGAGE CLAIM 8 to rescue zone WHO HADN'T SLEPT IN LIKE 40 HOURS. So then we got out of Portland, which I am now convinced was designed specifically to kill people, and it took like and hour and a half to get back home and we JUST BARELY got to the apartment before midnight and then we played games for two hours and passed out. And then we woke up like every 3 hours and SAM'S FUCKING SNORING KEPT WAKING ME UP OH MY GOD but eventually we sort of all got to sleep. Also the bathroom door squeaks.

July 6, 2011

Radians are used in almost everything that involves computers, and often come up in mathematical equations. However, most people are used to measuring angles in degrees, even though degrees (unlike radians) are a completely arbitrary measurement. To understand Radians, you need to understand $$\pi$$, or more precisely, why it sucks. Like a black hole, except black holes don't actually suck, they just warp space-time so you fall into themOK BACK ON TOPIC:

$$\pi$$ is the ratio of a circle's diameter to its circumference. This is retarded. When was the last time you thought of a circle using its diameter for crying out loud? We define circles, and should intuitively think of circles using their radius, which is the distance from the center to any point on the edge of the circle.

So that means that the ratio of the radius to the circumference is actually $$2\cdot\pi$$. This is why the formula for the circumference of a circle is $$2\cdot\pi\cdot r$$. In fact, its why we end up using $$2\cdot\pi$$ almost everywhere, because we almost always define circles with the radius, but $$\pi$$ is in relation to the diameter. Which is just stupid, and is in fact so stupid that some people are currently trying to define $$\tau$$ (Tau) such that $$\tau = 2\cdot\pi$$. Sadly, $$2\pi$$ still rules the world. I guess mathematicians really like pie...?

Either way, we must work within our constaints. So, keeping in mind that $$2\pi$$ is the ratio between a circle's circumference and its radius, what happens if you have a unit circle (a circle with a radius of 1)? Well, that means it has a diameter of 2, so if you roll it out on the number line, you get this:

When we talk about rotation and angles in radians, we are using the following relation:
\[ 360^{\circ} = 2\pi\;rad \]That is, in radians, or $$rad$$ as it is commonly abbreviated, a full rotation is $$2\pi$$ because that's the circumference of a circle with a radius of 1. Radius → Radians. We know why its $$2\pi$$ instead of just $$pi$$, because $$pi$$ is stupid. If you want, you can say that $$\tau = 2\pi$$, so that $$\tau$$ is then equal to the ratio between a circle's radius and its circumference. Then we get the much nicer relation:
\[ 360^{\circ} = \tau\;rad \]360° is just one rotation of a circle. A radian is simply how far around the circumference of a unit circle you want to go. So that means 180° is halfway around the circle, which is $$\tau/2$$ or just $$\pi$$ radians.

Degrees

Radians

360°

$$2\pi$$

180°

$$\pi$$

90°

$$\frac{\pi}{2}$$

60°

$$\frac{\pi}{3}$$

45°

$$\frac{\pi}{4}$$

30°

$$\frac{\pi}{6}$$

This gives rise to the conversion functions:
\[ \begin{aligned}
rad = deg \cdot \left(\frac{\pi}{180}\right) \\
deg = rad \cdot \left(\frac{180}{\pi}\right)
\end{aligned}
\] But these simply arise out of $$2\pi$$ being simplified:
\[ \begin{aligned}
rad = deg \cdot \left(\frac{2\pi}{360}\right) \\
deg = rad \cdot \left(\frac{360}{2\pi}\right)
\end{aligned}
\] So now we have an intuitive explanation of what radians are - the distance around the circumference of the unit circle.

When moving from C# to C++, one must have a very deep knowledge of what C# is actually doing when you run your program. Doing so allows you to recognize the close parallels between both languages, and why and how they are different. This tutorial will assume you have a fairly strong grasp of C#, but may not be familiar with some of its more arcane attributes.

In C#, everything is an object, or a static member of an object. You can't have a function just floating around willy-nilly. However, like all programs, a C# program must have an entry-point. If you have primarily done GUI-based design, you probably aren't aware of the entry-point that is automatically generated, but it is definitely there, and like everything else, it's part of an object. C# actually allows you to change the entry point function, but a default C# project will automatically generate a Program.cs file that looks like this:

static void Main() is the real entry point for your application, which simply initializes visual styles and then immediately launches the form that most C# users are accustomed to working with. Now, we can compare this with a simple "Hello World" C++ program:

This program, to a C# user, immediately looks foreign and possibly even outright hostile. However, almost everything in it has a direct analogue in C#, despite the rather inane syntax that is being used. The most glaring example here is the insertion operator, <
#include <iostream>
int main(int argc, char *argv[])
{
std::cout.write("Hello World",11);
return 0;
}
Now, counting the number of bytes you are pumping into the stream is really annoying, and that's what the insertion operator does for you; it properly formats everything automatically. That's all. It's not a demon from hell bent on destroying your life, its just weird syntax. I don't know why they don't also have this functionality in a much easier to understand overloaded function, but there are a lot of things that they don't do, so we'll just have to live with it.

The main() function here serves the same exact purpose as the Main() function in C#. Strict C++ requires you to have a main() function to serve as an entry point, but various operating systems modify it and, in the case of Windows, outright replace it. As such, you will notice that your "hello world" C++ program, when built, opens in a command line. You will learn later how to prevent this by using Windows' proprietary entry function. For those of you familiar with C#, this is exactly the same as C#'s ability to change around the entry point of the application, and you can even make a command line application in C# too by properly changing the compiler settings. The same concept applies to C++, but unlike C#, which defaults to a GUI, C++ defaults to a command line. Changing the compiler settings properly will result in a C++ program that starts in a GUI, just like C# (although unlike C#, C++ doesn't have any help, which turns GUI programming into a complete nightmare).

So now that we have a direct analogue between C# and C++ in terms of where our application starts, we need to deal with a conceptual difference in how C# and C++ handle dependencies. In C#, your class file is just Class.cs, your helper class is Helper.cs, and both of them can call the other one provided they are in the same namespace, or if you are inheriting someone else's, using the correct using statements to resolve the code. If these concepts are not familiar to you, you should learn more about C# before delving further into C++.

C++, on the other hand, does not do behind-the-scenes magic to help you resolve your dependencies. To understand what C++ is doing, one must understand how any compiler resolves references inside code (including C#). When the C# compiler is compiling your project, it goes through each of your code files one by one and compiles everything to an intermediate object code that will later be compiled down into the machine code (or, in this case, bytecode, since C# is an interpreted language). But wait, what if it's compiling Class.cs before Helper.cs even though Class instantiates a Helper object and calls some functions inside of it that then instantiate another Class object? Well, what if you compiled Helper.cs first... but Helper.cs needs Class.cs to be compiled first because its instantiating a Class object inside the function that the Class object is calling! That's a circular dependency! THIS IS IMPOSSIBLE OH GOD WE'RE GOING TO DIE No, it's actually quite simple to deal with. Enter prototypes. If you have the following C# class:

Notice the distinct lack of code - this is how circular references get resolved. It turns out that to properly compile your program, the compiler only has to know what a function takes in as arguments, and what it returns. By treating the function as a "black box" of sorts, the compiler can ignore whatever code is inside it. Notice that this applies to constructors and destructors as well - they are simply special functions inside the class. In this manner the entire class can be treated as a bunch of black-box functions that don't actually have any code that needs to be compiled in them. What the C# compiler does is create a bunch of these prototypes behind the scenes and feed them in front of all your code files, so it first compiles Class.cs using a prototype of the Helper class, which allows it to instantiate and use any functions that Helper defines without actually knowing the code inside them. Then, it compiles Helper.cs, compiling assigning code to the previously empty black-box functions defined in the Helper prototype, using a prototype of Class so that it can also instantiate and call functions from Class. In this way, both Helper.cs and Class.cs can be compiled in any order.

But wait, what if Class inherits Helper? In reality, this changes nothing. An important lesson here is that, in C++, you will not be able to simply ignore the fact that everything is a function. Classes are just an abstraction - in reality, inheritance, constructors, deconstructors, operators, everything is just various special functions. Python's class syntax is interesting because requires that all class functions explicitly define the self parameter (which is identical to the this reference in C++ and C#), even the class constructor. Both C++ and C# hide all this from you, so Constructors and Destructors and class functions all magically just work, even though underneath it all they're just ordinary functions with a special parameter that's hidden from view. This is, in fact, how one mimics class behavior in C, which does not have object-oriented features - simply build a struct and make a bunch of functions for it that take a "this" pointer, or a pointer to a specific struct on which the function operates. This behavior can be (needlessly) duplicated using C# - let's transform our Class class to C-style function implementations, ignoring the slightly invalid C# syntax.

Thankfully, we don't have to worry about this, since thinking of class functions as functions that operate on the object is a lot more intuitive. However, one must be aware that even in inheritance scenarios, everything is just a function, or an overload of a virtual function, or something similar (if you do not know what virtual functions are, you need to learn more C# before proceeding). Consequently, our ability to declare function prototypes solves all the dependency issues, because everything is a function.

This is where we get into exactly what the #include directive is for. In C#, all your files are automatically accessible from every other file, and this isn't a problem because compilation is nigh-instantaneous. C++ is much more intensive to compile, partially because it doesn't have a precompiled 400 MB library of crap to work off of, and partially due to a much more complicated precompiler. That means in C++, if you want a given file to have access to another file, you have to #include that file. In our Hello World application, we are including iostream, which does not have a .h file extension on the end for stupid regulatory reasons. However, what about the file our code is in? Our code is not in a .h file, its in a .cpp file. This is where we get to a critical difference between C# and C++. While C# just has .cs files for code, C++ has two types of files: header files and code files.

.cpp == C++ (C-plus-plus) code file
.h = C++ Header file

Header files contain class and function prototypes, and code files contain all the actual code. A C++ project is therefore defined entirely by a list of .cpp files that need to be compiled. Header files are just little helper files that make resolving dependencies easier. C# does this for you - C++ does not. Note that because these are technically arbitrary file distinctions, you can put whatever you want in either file type; nothing will stop you from doing #include "main.cpp", its just ridiculous and confusing. Both #include <> and #include "" are valid syntax for the #include directive, there is no real difference. Standard procedure, however, is that #include <> is used for any header files outside of your project, and #include "" is used for header files inside your project, or closely related to it.

So what we're doing when we say #include <iostream> is that we're including a bunch of prototypes for various input/output stream (i/o stream --> iostream) related classes defined in the standard library, which your compiler already has the corresponding .cpp implementations of built into it. So, the compiler links the application against this header file, and when you use std::cout, it just treats everything in it (including that ridiculously obtuse << operator, which is really just another function) as a black-box function.

Consequently, unless you know what your doing, you should keep code out of header files. C++ doesn't prevent you from throwing functions that aren't attached to classes all over the place, like C# does, so what would happen if you defined int ponies() { return 0; } in a header file that you include in two seperate .cpp files? The compiler will try to compile the function twice, and on the second time it will explode because the function it tried to put code into already had code in it, since it wasn't a prototype! EVERYTHING DIES! So until you get to the more advanced areas of C++, don't put code in your header files (unless you want to watch your compiler die, you monster).

At this point I want to clarify what std:: is, because it looks rather weird to a C# programmer. In C#, the . operator works on everything - you just have System.Forms.Whatever.Help.Im.Trapped.In.A.Universe.Factory.Your.Class.Member.Function() and its all good. In C++, that's not going to work anymore. The :: operator is known as the Scope Resolution Operator. It's a lot easier to explain if I first explain what the . operator has been demoted to. You can only use the . operator on a reference or value type of an instantiated object (basically everything you've ever worked with in C#). The important distinction here is that static functions cannot be accessed with the . operator anymore. This is because Static functions, along with namespaces and typedefs and everything else must use the Scope Resolution Operator. Consequently, you can think of the . operator as being demoted to just calling class functions, and everything else now uses the :: operator. So, std::cout just means that we're access the cout class in the std namespace.

Now we just have one more hurdle to overcome with the "Hello World" application, that funky char* argv[] parameter in main(). Most C# programmers can correctly infer that it is probably an array of some sort, but we don't know what type char* is, other than its clearly related to char.

char* is a pointer. Yes, the same scary pointers you hear about all the time. No, they aren't really scary. In fact, you have been using similar concepts in C# all the time without actually realizing it. First, however, let's take a hard look at what a pointer really is.

Everything in your entire program takes up memory. Since this tutorial is designed for people who know C# already, I really, really hope you already knew that. What you might not know is that all this memory has a specific location on the machine. In fact, on a 32-bit machine, every single possible location of a byte can be contained in an unsigned 32-bit integer. This is why we are currently moving to 64-bit CPUs, because an unsigned 32-bit integer can only hold up to 4294967295 possible byte locations, which amounts to 4.2 gigs of memory. That's why you are limited to 4 gigs of RAM on a 32-bit machine, and windows has difficulty using more than 2 gigs because a lot of older programs assumed that a signed 32-bit integer was sufficient for all memory addresses, so windows has to do some funky memory paging techniques to get programs that ignore the last bit to use memory locations above 2147483647.

So, if you allocate a float, either on the stack or on the heap, it must exist somewhere within those 4294967295 possible byte locations. Consequently, lets say you want to call a function that modifies that float, but the function has to have a void return value for some arbitrary reason. If you know where in memory that float is, you can tell the function where to find the float and modify it to the desired value without ever returning a value. Here is an example C++ function doing just that (which is syntactically valid all by itself because C++ allows functions outside of classes):

What's going on here? First, we have our ModifyFloat() function. This takes a pointer to a float, which is declared by adding a * to the desired type we want to make a pointer to. Remember that pointers are really just 32-bit integers (or 64-bit if you have a 64-bit operating system), but C++ assigns them a type so that if you try to assign a double to a pointer to a float, it throws an error instead of overflowing 4 extra bytes, causing a heap corruption and destroying the universe. So char* is a pointer to a char, a double* points to a double, and Helper* is a pointer to our own Helper class.

The next thing done in ModifyFloat() is *p. In this case, the * operator is the dereference operator. So unfortunately * is the multiply, pointer, and dereference operator in C++. Yes, this is retarded. I'm sorry. But what the heck does dereference even mean? It takes a pointer type and turns it into a reference. You already know what a reference is, even if you don't realize it. In C#, you can pass a variable of your Helper class into a function, modify the class in the function, and the original variable will get modified too! This is because, by default, classes are passed by-reference in C#. That means, even though it looks identical to a variable passed by value, any changes made to the variable are in fact made to whatever variable it references. So, this idea of passing variables in by reference should be familiar to an experienced C# programmer. C++ has references too, I just haven't gone over their syntax. Here's a more explicit version of the function:

void ModifyFloat(float* p)
{
float& ref_p = *p;
ref_p = 100.0;
}

This is the exact same as the previous function, but here we can clearly see the reference. In C#, if you wanted a variable normally passed by value, like a struct, to get passed by reference, you had to override the default behavior by adding ref. In C++, a variable that is a reference to a given type is declared in a similar manner to a pointer. The & operator is used instead of *, so in this example, float& is a reference to a float. We assign it to the value produced by turning our pointer into a float reference. Then we just set our reference equal to 100.0 and it magically alters the original variable, just like it would in C#. In fact, here is the same function written in (slightly illegal) C#:

public static void ModifyFloat(ref float p)
{
p=100.0;
}

This does the same thing, just without the pointer. In fact, we can totally ignore the pointer in C++ too, if we want (which I tend to prefer, when possible, because its a lot easier to work with):

Now, in this implementation, you will notice that our call to ModifyFloat is now equivalent to what it would be in C#, in that we just pass in the variable. What happened to that random & operator we had there before? The & operator is also known as the address-of operator, meaning when its applied to a variable as opposed to a type, it returns a pointer to that variable (yay, more context-dependent redundant operators). So, we could rewrite our function as follows to make it a bit more clear:

As we can see, pointers are just the underlying work behind references. If you ever go into Managed C++, you'll find out that all C# references are really just pointers, but the language treats them as references so they're hidden from you. In C++, you can have both pointers and references. It is important to note that you can only initialize a reference variable. Any subsequent operators will be applied to whatever variable its referencing, making it impossible to get the address of a reference variable or do anything to the reference variable itself - for all intents and purposes, it just is the variable it references. This is why pointers are handy - you CAN reassign the actual pointer variable while also accessing the variable its pointing to. Consequently, you can also get the address of a pointer variable, since just like any other variable, including the reference variable, it must occupy memory, and therefore has a location that you can get a pointer to (we'll get to that syntax in a minute). But there's one more thing...

What about arrays? In C#, arrays are actually a built-in class that has lots of fancy functions and whatnot. Interestingly, they are still of fixed size. C++ arrays are also fixed size, but they are manipulated as raw memory. Let's compare initializing an array in C++ and initializing an array in C#:

int[] numbers = new int[5];

int* numbers = new int[5];

It should be pretty obvious at this point that arrays are pointers in C++. I can even rewrite the above in C++ using an empty array syntax, and it will be equally as valid:

int numbers[] = new int[5];

int x[] is identical to int* x. There is no difference. Observe the following modification of our original Hello World function:

int main(int argc, char** argv);

Same thing. In fact, if you watch your compiler output carefully, you might even see the compiler internally convert all the arrays to pointers when its resolving types. Now, as a C# programmer, you should already know what arrays are. You should probably also be at least dimly aware that each element of an array occupies memory directly after the element proceeding it. So, if you know where the address of the first element is, you know the second element is exactly x bytes afterwards, where x is the number of bytes your type takes up. This is why pointers have types associated with them - we know that float* points to an array of elements, and that each element takes up 4 bytes. To verify this, the sizeof() built-in function/operator/whatever will return the number of bytes a given type, class, or struct takes up. That's the number of bytes we skip ahead to get to the next element in an array. This is all done transparently in C++ using the same array index operator as C# uses:

So pointers can be treated as arrays that behave exactly the same way a C# array does. However, the astute C# programmer would ask, how do you know how long the array is?

YOU DON'T

Enter every single buffer overflow error that has been the bane of man since the beginning of time. YOU have to keep track of how long the array is, and you'd better be damn sure you don't get it wrong. Consequently any function taking an array of variable size will also require a separate argument telling the function how many elements are in the array. Usually arrays are just constructed on the stack with a constant, known size, which is often harmless and pretty hard to screw up. If you start doing funky things with them, though, you might want to look up std::vector for an encapsulated dynamic array.

So C++ arrays are just like C# arrays, except they are pointers to the first element, and you don't know how long they are (and they might cause the destruction of the universe if you screw up). You should already know that a string is an array, and consequently in C++ the standard string type is const char*, not string. You also can't put them in switch() statements. Sorry.

There's a lot of stuff about pointers that this tutorial hasn't covered, like function pointers and pointer arithmetic, which we'll get to next time.